package com.ryzx.id.generator;


import org.springframework.beans.factory.annotation.Value;
import org.springframework.stereotype.Service;

@Service
public class IDGenerator {

    //10bit的工作机器id
    @Value("${custom.workerId}")
    private static long workerId;    // 5bit
    @Value("${custom.datacenterId}")
    private static long datacenterId;   // 5bit

    private static final long twepoch = 1609430400000L;           //初始时间戳  2021-1-1 0:0:0

    private static long sequence=1; // 12bit 序列号

    //长度为5位
    private static final long workerIdBits = 5L;
    private static final long datacenterIdBits = 5L;
    //最大值 -1 左移 5，得结果a，-1 异或 a：利用位运算计算出5位能表示的最大正整数是多少。
    private static final long maxWorkerId = ~(-1L << workerIdBits); //31
    private static final long maxDatacenterId = ~(-1L << datacenterIdBits); // 31
    //序列号id长度
    private static final long sequenceBits = 12L;
    //序列号最大值
    private static final long sequenceMask = ~(-1L << sequenceBits); //4095

    //workerId需要左移的位数，12位
    private static final long workerIdShift = sequenceBits; //12
    //datacenterId需要左移位数
    private static final long datacenterIdShift = sequenceBits + workerIdBits; // 12+5=17
    //时间戳需要左移位数
    private static final long timestampLeftShift = sequenceBits + workerIdBits + datacenterIdBits; // 12+5+5=22

    //上次时间戳，初始值为负数
    private static long lastTimestamp = -1L;

    public long getWorkerId(){
        return workerId;
    }

    public long getDatacenterId(){
        return datacenterId;
    }

    public long getTimestamp(){
        return System.currentTimeMillis();
    }

    /**
     * 返回可用ID
     * @return 20位数字ID
     */
    public static synchronized long nextId() {
        long timestamp = timeGen();

        //获取当前时间戳如果小于上次时间戳，则表示时间戳获取出现异常
        if (timestamp < lastTimestamp) {
            System.err.printf("clock is moving backwards.  Rejecting requests until %d.", lastTimestamp);
            throw new RuntimeException(String.format("Clock moved backwards.  Refusing to generate id for %d milliseconds",
                    lastTimestamp - timestamp));
        }

        //获取当前时间戳如果等于上次时间戳（同一毫秒内），则在序列号加一；否则序列号赋值为0，从0开始。
        if (lastTimestamp == timestamp) {
            // 通过位与运算保证计算的结果范围始终是 0-4095
            sequence = (sequence + 1) & sequenceMask;
            if (sequence == 0) {
                timestamp = tilNextMillis(lastTimestamp);
            }
        } else {
            sequence = 0;
        }

        //将上次时间戳值刷新
        lastTimestamp = timestamp;

        /*
         * 返回结果：
         * (timestamp - twepoch) << timestampLeftShift) 表示将时间戳减去初始时间戳，再左移相应位数
         * (datacenterId << datacenterIdShift) 表示将数据id左移相应位数
         * (workerId << workerIdShift) 表示将工作id左移相应位数
         * | 是按位或运算符，例如：x | y，只有当x，y都为0的时候结果才为0，其它情况结果都为1。
         * 因为个部分只有相应位上的值有意义，其它位上都是0，所以将各部分的值进行 | 运算就能得到最终拼接好的id
         */

        return ((timestamp - twepoch) << timestampLeftShift) |
                (datacenterId << datacenterIdShift) |
                (workerId << workerIdShift) |
                sequence;
    }

    //获取时间戳，并与上次时间戳比较
    private static long tilNextMillis(long lastTimestamp) {
        long timestamp = timeGen();
        while (timestamp <= lastTimestamp) {
            timestamp = timeGen();
        }
        return timestamp;
    }

    //获取系统时间戳
    private static long timeGen(){
        return System.currentTimeMillis();
    }

    //---------------测试---------------
    public static void main(String[] args) {

        System.out.println(workerId);
        System.out.println(datacenterId);
        for (int i = 0; i < 30; i++) {
            System.out.println(IDGenerator.nextId());
        }
    }

}